In optical wavelength division multiplexed communication systems (WDM systems), an optical fiber simultaneously carries many different communications channels in light of respectively different wavelengths. In WDM systems it is desirable to ensure that all channels have nearly equivalent signal-to-noise ratios. To help achieve this equation, gain equalization filters are disposed at various points along the system to control the relative power levels in respective channels.
Dense WDM systems require special add/drop multiplexer filters (ADM filters) to add and drop particular channels (wavelengths). At each node in the system, certain wavelengths on one fiber must be dropped onto a second fiber and channels from a third fiber must be added. The filters to accomplish this task advantageously have sharp roll-off characteristics to minimize cross-talk between channels and interference between an incompletely dropped channel and an added channel. Flat passband characteristics are also desirable since they tolerate more variation in the source wavelength and maintain a finite passband width after concatenating many devices.
A proposed ADM filter using Bragg gratings in Mach-Zehnder interferometers ("MZIs") is described by T. Erdogan et al in "Integrated Optical Mach-Zehnder Add-Drop Filter Fabricated by a Single UV-Induced Grating Exposure", Proceedings of Optical Fiber Conference, San Jose, Calif. (February 1996). In essence the device comprises a plurality of concatenated MZIs (one stage for each add/drop wavelength), each MZI including a Bragg grating in each arm.
While the proposed Erdogan et al. filter has advantages of passband flatness, sharp rolloffs and a high rejection in the passband, it suffers excess loss at shorter wavelengths. Such loss is due to coupling to radiation modes. Short wavelength loss becomes a limiting factor when the wavelength range covered by the system channels becomes large. Longer gratings are required to ensure that dropped wavelengths do not interfere with subsequently added channels in the concatenated system. This increased length is undesirable in planar waveguide implementations. Accordingly there is a need for an improved switchable filter.